However, many alloys, such as austenitic stainless steels flange, duplex stainless steels, nickel alloys and titanium, are more resistant to uniform corrosion and tend to corrode locally — that is, pit or crack. So, it's less appropriate to specify a corrosion allowance for these materials in relatively benign processes.
Furthermore, as the thickness of the stainless steel increases, the more likely it can become sensitized from repeated heat input during multi-pass welding. While a mere 1/8-in. corrosion allowance doesn't seem like much, it potentially can require a disproportional number of additional weld passes depending on the weld procedure used.
A corrosion allowance isn't recommended for materials that are susceptible to stress corrosion cracking in a given process. For example, for protection against chloride-induced stress corrosion cracking, it would be more appropriate to upgrade the material of construction to a duplex, lean duplex or super duplex stainless steel, rather than add a corrosion allowance to austenitic stainless steel. Specifying some duplex alloys actually can provide a cost savings because they have 20% to 35% higher allowable code stresses, resulting in a thinner wall vessel
In summary, when uniform corrosion is expected, specify a corrosion allowance. When localized corrosion is expected, investigate other corrosion protection schemes.
Know the relative cost of materials. It's common knowledge that stainless steel is more expensive than carbon steel. However, the cost difference between a 300-series stainless steel and a lean duplex, duplex or super duplex stainless steel — or between stainless steel and high nickel alloys or zirconium — is less obvious. It's helpful to have a rough idea of the relative costs of materials so that meaningful discussions can take place during project development Table 2 lists the relative costs of commonly used materials. Note, though, the cost and delivery for any given alloy can vary greatly from vendor to vendor based on current stock and availability. It's always good practice to question the fabricator about how many material suppliers it got quotes from or to make independent inquiries into material costs, especially if you intend to sole source.
Keep critical metal temperatures in mind. We learn at an early age that water boils at 212°F (100°C) and freezes at 32°F (0°C) at atmospheric pressure. Engineers know that as water crosses these points, its physical and thermodynamic properties change and a new set of conditions apply. Many engineers, however, don't appreciate that solids also have temperature limits that, when crossed, create problems for the designer and thus can add additional steps to the fabrication process. The most common material limit occurs at low temperature and is called the ductile-to-brittle transition temperature.
It's an issue with carbon steels and other metals with a body centered cubic structure and manifests as a loss of ductility, the metal becomes brittle. Stainless steels, nickel-base alloys, aluminum, and copper also have limits but at temperatures below —325°F. Carbon steels's ductility decreases with temperature and as carbon content increases. The ASME code protects against brittle failures by limiting carbon content to no more than 0.35% and by mandating the material either to be heat treated or impact tested when the ductile-to-brittle transition zone is approached. For thin-wall vessels, common carbon steel materials (SA-105, SA-106, SA-516-70) require heat treatment or impact testing at design temperatures below -20°F. So, when possible, specify a warmer minimum design metal temperature to avoid these costs.
Proper selection of a coating that will resist the process is key. Coatings have limitations, primarily temperature. Many are restricted to 200°F to 300°F; they have a different coefficient of thermal expansion than the base metal they cover, which may make them more susceptible to separation over their service life. Like metallic vessels, coated equipment also requires periodic inspections. However, for moderate design temperatures, coating a carbon-steel vessel can be much more economical than purchasing a high alloy vessel or clad carbon-steel vessel. For instance, estimates for ethanol plants show savings of as much as 35% for coated carbon steel tanks compared to stainless ones.
Option 1 is to use the extra metal to rate the vessel with a higher Maximum Allowable Working Pressure (MAWP) than the required design pressure, 178 psig instead of 150 psig here. This choice favors continuous processes, and gives production the option to operate the vessel harder.
Option 2 is to set the MAWP equal to design and use the extra metal as additional corrosion allowance. This will give you a longer service life, which favors batch processes.
Option 3 is to set MAWP equal to design and use the extra metal to obtain a higher maximum design temperature. This option favors processes that have automatic temperature trips, such as exothermic reactors and fired heaters, and avoids possible fitness-for-service determinations if an excursion should occur.
The option selected can be changed later by performing a re-rate, although choosing Option 2 or 3 would require a new hydrostatic test. Also, when opting for Option 1 or 3, watch crossing into the next higher flange class.
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